The present invention relates in general to a method and apparatus for accomplishing signal companding. In particular, the present invention is directed to a method and apparatus that employs log-offset compression to accomplish signal companding. The invention is particularly useful in video signal processing circuitry for HDTV Telecine machines, video cameras and the like.
Telecine machines are devices which are used to convert images present on motion picture film to a video image signal. One of the most popular type of telecine machine employs CCD line scanners and digital signal processing techniques to scan and convert the film image. The popularity of telecine machines employing CCD scanners and digital signal processing is due in part to the high reliability attributable to the CCD devices and the high picture quality which can be obtained through the use of digital signal processing techniques.
The application of CCD line scanners to HDTV telecine machines, however, present special problems due to the substantially higher signal bandwidth required by HDTV systems. For example, CCD telecine machines typically employ one or more CCD sensors featuring an imaging array of 1728 and 2048 pixels and requiring clock rates in excess of 50 MHz. Unfortunately, the digitization of a video signal having the high bandwidth signal and dynamic range associated with HDTV requires analog-to-digital (A/D) converters beyond the capability of those currently available. Current state of the art monolithic A/D converters, however, are limited to an operating range of about a nine bit resolutions sampling at 30 Msamp/sec.; far below what is needed in the implementation of a digital CCD telecine machine. Higher resolution capability is therefore required in order to reduce the effects of quantization noise at low signal amplitudes.
Signal companding can be employed to overcome the quantization limitations of currently available A/D converters. Signal companding prior to digitizing permits the quantization steps to be tapered to provide a uniform signal to noise (S/N) ratio over a larger dynamic range than would be possible if the signal were not compressed. An example of such quantizing "pre-correction" is mentioned, for example, in a paper entitled "Digital CCD Telecine for HDTV" by Dieter Poetsch, which was presented at the International TV Symposium in Montreux, Switzerland, June 1989.
Logarithmic amplifiers are commonly used in video systems for scanning film and in many applications where a large dynamic range is required. There are disadvantages, however, associated with the use of logarithmic amplifiers in signal companding circuits for HDTV telecine machines. Logarithmic amplifiers are extremely sensitive to thermal changes at low signal levels and thus require a stable temperature environment or a temperature compensation network. To some extent, temperature problems associated with logarithmic amplifiers have been compensated for by keeping the logging transistor in thermal equilibrium using a black feedback clamp to adjust the logging transistor's current to zero during the black reference period of the video signal. In addition to temperature sensitivity, however, logarithmic amplifiers also commonly have bandwidth or settling time problems at low signal levels.
The problems associated with logarithmic amplifiers at low signal levels is particularly detrimental when attempting signal companding in telecine machines that scan print film. Print film has a higher density and a larger dynamic range than negative film making it more difficult to provide the bandwidth required (on the order of three decades) and to maintain the log transfer function due to temperature instabilities.
Accordingly, it would be desirable to develop a method and apparatus for signal companding that provides a high bandwidth over the dynamic range of the signal as well as temperature stability. Further, it would be desirable to develop a method and apparatus for signal companding that could be adjusted to provide different transfer curves based on the type of film being scanned.